Fluorine-containing salts are known as compounds having unique characteristics, such as light transmitting property. In particular, complex salts (salts containing a plurality of cationic species) are useful as light-emitting materials, ferroelectric materials, etc. Non-patent Publication 1 describes complex fluorides, such as ion conductors (e.g., PbSnF4), magnetic materials (e.g., Ba7CuFe6F34), light-emitting materials (e.g., 4f-metal containing KMgF3, BaLiF3, and LiYF4), ferroelectric materials (e.g., SrAlF5 and BaMgF4), etc. Non-patent Publication 2 also describes complex oxyfluoride ferroelectrics, such as K3MoO3F3 and Bi2TiO4F2, together with complex fluorides, such as BaMF4 (M=Mg, Mn, Fe, Co, Ni or Zn), SrMF5 (M=Al, Cr or Ga), and BaMF5 (M=Ti, V, Fe, or In). Compounds having a structure of LiAIIMIIIF6 (A=metal species to become a bivalent cation(s), such as Mg, Ca, Sr, Ba, Ni, Cu, Zn, Cd, Hg, etc., and M=metal species to become a trivalent cation(s), such as Al, Ti, V, Cr, Mn, Fe, Co, Ni, etc.) are also known. The application to optical materials as shown in Patent Publication 1 and the application to cathode materials of lithium ion batteries as shown in Patent Publication 2 are examined. Furthermore, ferroelectric complex fluoride crystals, such as BaMgF4, BaZnF4, SrAlF5, Na2MgAlF7, and Na2ZnAlF7, are mentioned, for example, in Patent Publication 3 as materials of wavelength conversion devices using nonlinear optical crystals, since they are highly transparent even in an ultraviolet region of 200 nm or shorter.
In case that a plurality of cations constituting a complex salt have similar ionic radii, a solid solution can be formed by occupying the same site in the crystals. In case that the difference of ionic radii becomes large, however, the formation of a solid solution in a wide range becomes difficult, resulting in occupying different sites in the crystals. For example, as shown in Non-patent Publication 2, in the case of BaMF4 (M=Mg, Mn, Fe, Co, Ni or Zn), Ba occupies an 8-coordinated site, and M occupies a 6-coordinated site. Fluorine surrounding M takes a distorted octahedron structure, which connects with adjacent octahedron in the form of holding vertexes in common. By the rotation of this octahedron, the position occupied by Ba shifts to generate polarization inversion. Therefore, BaMF4 shows ferroelectricity. Furthermore, as mentioned in Non-patent Publication 3, it is known that, since the lattice defects of F sites of BaMgF4 well trap electrons, a superior light-emitting property is shown by doping with Eu or the like. Upon this, Eu selectively occupies Ba sites having a similar ionic radius. Furthermore, Non-patent Publication 4 shows an example of observation of the energy transfer from Eu2+ to Mn2+ by simultaneously doping BaMgF4 with Eu and Mn. In this case, they selectively occupy sites having similar ionic radii, as Eu occupies Ba sites and as Mn occupies Mg sites.
Furthermore, Non-patent Publication 5 shows various crystal structures of M1nM2mM3F6 type complex fluorides. Of these, for example, LiCaAlF6 (LiCAF) and LiSrAlF6 (LiSAF) are known to become laser materials by doping with Ce, Cr, etc. As shown in Non-patent Publication 6, it is known that both of LiCAF and LiSAF take a Colquiriite-type crystal structure, in which the layer of 6-coordinated Al and Li and the layer of 6-coordinated Ca (or Sr) are alternately laminated.
Thus, complex salts, particularly fluorine-containing complex salts that contain at least two cation-species groups not capable of forming a solid solution, respective groups occupying crystallographically distinct sites, are rich in functionality and contain many useful substances. Their syntheses are, however, not easy.
As processes for synthesizing fluorine-containing complex salts, there are known processes in which the syntheses are conducted in a gas atmosphere of fluorine or hydrogen fluoride or in a liquid of anhydrous hydrogen fluoride by using a solid mixture of single-cation fluorides, single-cation chlorides, single-cation nitrates, etc. as cation sources. In these processes, fluorine, hydrofluoric acid or hydrogen fluoride, which is highly reactive chemical species, is used as a reactant. Therefore, it has been difficult to control the atmosphere, remove an excess of the reactant from the product, or prevent corrosion of the production equipment. Furthermore, in the case of synthesizing a fluorine-containing complex salt by conducting a solid-phase mixing of single-cation fluorides and then calcination, there has been a defect that the unreacted, single-cation fluorides remain, since it is difficult to complete the solid-phase reaction.
In contrast with these conventional processes, there are proposed a sol-gel process using trifluoroacetic acid, and a one-step process by undergoing an aqueous solution phase. In the sol-gel process shown in Non-patent Publication 7, the constituent cations of fluorine-containing salts and trifluoroacetic acid are once dissolved in a solvent, followed by mixing. Then, a gel is obtained by distilling the solvent off by a step of concentration and drying and under certain circumstances by a preliminary calcination step of 200° C. This gel is pulverized and then calcined to produce a fluorine-containing salt in the end. Fluorine is generated by a thermal decomposition of trifluoroacetate anion. Since this decomposition temperature is around 300° C., it is understood that the reaction of synthesizing the fluorine-containing salt proceeds by the calcination step. Furthermore, in the one-step process shown in Non-patent Publication 8, the constituent cations of the target salt are dissolved in an aqueous solution, followed by adding hydrogen fluoride, thereby conducting the reaction and synthesizing a fluorine-containing salt. In each example, there is reported the synthesis of fluorine-containing complex salts, such as BaMgF4, SrAlF5, and LiCaAlF6.